The proposed investigation is concerned with the correlation of alterations in cell structure and function with age-associated mortality rates of the houseflies. The major goal is to identify cellular aging changes which are specifically associated with the physiological rather than the chronological rate of aging. Distinction between the physiological and the chronological age of the animals will be made by experimental manipulations of the life spans. The life spans will be varied experimentally by altering the metabolic rate, and genetically by selective breeding of short- and long-lived lines of houseflies. Life span of the housefly is inversely related to metabolic rate. The average and the maximum life spans can be prolonged approximately two-fold by restriction of the flight activity. The effects of experimentally altered life spans on metabolic rate (oxygen consumption), activities of p-phenylenediamine-mediated peroxidase, catalase, and Beta-glucoronidase, concentration of malonaldehyde, lipofuscin content, secretory activities of Malpighian tubules and fine structure of postmitotic cells in the housefly will be studied. The results of this study should indicate if cellular alterations in the above parameters respond to the experimental variations in life span, and also reveal the effects of lipofuscin accumulation on the functional capacity of tissues. The potential usefulness of this study lies in the identification of physiologically significant cellular alterations accompanying aging.